The document discusses cellular transport and the plasma membrane. It describes the key functions of the plasma membrane including absorption and circulation of materials through the organism. The plasma membrane is selectively permeable and uses both passive and active transport mechanisms. Passive transport includes diffusion, facilitated diffusion, and osmosis which move molecules down concentration gradients without energy. Active transport uses ATP and transport proteins to move molecules against concentration gradients. Endocytosis and exocytosis are described as mechanisms to transport larger particles and molecules into and out of cells.